The invention concerns a rain sensor according to the general class of the independent claim. Numerous rain sensors have already been made known, for example in DE 198 15 748 A1, that measure the wetting of the windshield of a motor vehicle with the aid of a transmitter and a receiver via the principle of total reflection. Said rain sensors comprise a light-conducting body that is designed substantially flat in shape and is arranged on the interior of the motor vehicle nearly parallel to the surface of the windshield.
The rain sensor according to the invention having the features of the main claim has the advantage that the respective focussing needs can be taken into account by means of various structures for directing and catching radiation, by way of which the sensitivity of the rain sensor is improved.
Advantageous further developments and improvements of the features indicated in the main claim result from the measures listed in the dependent claims.
It is particularly advantageous when the structures for directing the radiation toward the window pane are spherical in character, and the structures for catching the radiation are non-spherical in character. In this fashion, a maximum amount of light is directed toward the windshield and, simultaneously, only the radiation relevant to the measurement of precipitation is reflected by the windshield. This increases the size of the the measuring regions on the windshield, therefore increasing the signal-to-noise margin of the rain sensor signal.
If the structures for directing the radiation are formed as spherical segments, they are very easy to produce as spherical structures, in particular when the light-conducting body is an injection-molded part.
It is furthermore advantageous when the structures for catching the radiation comprise cylindrical segments, the cover surface of which is formed as a non-spherical lens. The cylindrical segments bring about a shield-like effect, so that only the desired radiation can pass through the light-conducting body to the receiver. The non-spherical lenses make it possible to move the receivers closer to the light-conducting body than would be possible with spherical lenses.
It is particularly advantageous when the concentrated beams meet outside of the geometrical center line of the cylindrical segment. Due to this squinting lens, the optical components such as transmitter and receiver can be moved even closer to the light-conducting body, which further reduces the proportions of stray illumination.
If the surface of the light-conducting body capable of being coupled to the window pane comprises a slight convex arch, air bubbles that can form in the coupling region when the light-conducting body is coupled to the window pane are advantageously prevented. Additionally, tension is produced over the entire light-conducting body during installation, with the effect that changes in shape due to temperature or ageing, for example, can be offset.
If multiple structures for directing and catching radiation are provided in each case that are arranged in pairs in each case, a plurality of measuring paths can be arranged on the narrowest space, and the dimensions of the rain sensor can be limited to a minimum. This is particularly advantageous because the rain sensor can be arranged on the windshield in such a fashion that it does not obstruct the driver""s vision.
It is particularly advantageous when four structures each are provided for directing and catching radiation that are arranged in such a fashion that the measuring paths form a parallelogram when projected onto the window pane. By means of such an arrangement, four measuring paths can be realized using two transmitters and two receivers, whereby sufficient space is available in the center of the parallelogram for arranging a further sensor or a heating device, for example. Additionally, due to the fact that each transmitter transmits in multiple measuring paths and each receiver receives from multiple directions, costly optical semiconductor components can be spared.
If the surface of the light-conducting body capable of being coupled to the window pane is at least partially provided with one component of a multiple-component adhesive, the light-conducting body can be installed quickly and securely to the window pane.